1. Field of the Invention
This invention relates to electrography and more particularly to a method for preparing carrier particles for use in magnetic brush dry development of electrostatic charge images.
2. The Prior Art
Electrography broadly includes various processes that involve forming and developing electrostatic charge patterns on surfaces, with or without the use of light. One method of dry electrographic development is the magnetic brush method which is widely used in electrographic document copying machines. It is disclosed, for example, in U.S. Pat. No. 3,003,462. The method of the present invention is useful in preparing the carrier particles for two-component developers used in the magnetic brush method. A two-component developer is a mixture of thermoplastic toner particles and of magnetic carrier particles, the latter being partially coated with an insulating resin.
In one useful embodiment, the two-component developer is attracted to a magnetic brush consisting of stationary magnets surrounded by a rotating cylindrical sleeve. By frictional contact with the resin-coated carrier particles the toner particles are triboelectrically charged and cling to the carrier particles, creating bristle-like formations of developer on the magnetic brush sleeve. In developing a charge pattern the brush is brought close to the charged surface. The oppositely charged toner particles are drawn away from the carrier particles on the magnetic brush by the more strongly charged electrostatic charge pattern, thus developing and making visible the charge pattern.
Although uncoated iron particles can be used as carriers in magnetic brush developers and although the high conductivity of uncoated iron particles is desirable because a conductive magnetic brush serves as a development electrode and improves the development of large solid areas in the image, nevertheless resin-coated carrier particles have often been preferred. One reason for resin-coating the carrier particles has been to improve the triboelectric charging of the toner particles. When a resin-coated carrier is used, the toner powder acquires a high, net electrical charge because of the frictional contact of the toner particles and the resin coating. This high net charge reduces the amount of toner throw-off, i.e., the loss of toner from the developer mix as it is agitated in the magnetic brush apparatus.
U.S. Pat. No. 3,795,617 describes the use of a vinylidene chloride copolymer as the resin coating for magnetic carrier particles. The coated particles described in the patent and the method of preparing them are quite useful. A problem has been, however, that when the carrier particles are coated with resin in the manner described in the patent, the coating insulates the iron particles so much that the conductivity of the particles is low. This causes fringing development and the solid area development suffers. Usually, only after a long break-in period of use in a copier developing station does the coating wear off sufficiently to improve the conductivity of the carrier and the solid area development.